Optical crosstalk may exist between neighboring photosensors in a pixel array of a solid state imager, such as a CCD or CMOS imager, for example. Optical crosstalk in imagers can bring about undesirable results in images that they produce. The undesirable results can become more pronounced as the density of pixels in imager arrays increases, and as pixel size decreases.
In an idealized photosensor, a photodiode for example, light enters only through the surface of the photodiode representing the region of the imager that directly receives the light stimulus. In reality, however, light that should be received by neighboring photosensors also enters the photodiode, in the form of stray light, through the sides of the photosensor, for example. Reflection and refraction within the photosensor structure can give rise to the stray light, which is referred to as “optical crosstalk.”
Optical crosstalk can manifest as blurring or reduction in contrast, for example, in images produced by a solid state imager. As noted above, image degradation can become more pronounced as pixel and device sizes are reduced. Degradation caused by optical crosstalk also is more conspicuous at longer wavelengths of light. Light at longer wavelengths penetrates more deeply into the silicon structure of a pixel, providing more opportunities for the light to be reflected or refracted away from its intended photosensor.
Problems associated with optical crosstalk have been addressed by adding light shields to imager structures. The light shields are formed in layers fabricated above the admitting surface through which the photosensor directly receives light stimulus. The light shield layers generally include metal and other opaque materials. The light shields, however, often reflect a significant amount of light that may still cause optical crosstalk.
Generally, the added light shields are formed as part of the uppermost layers of the imager array. Light shields have been formed, for example, in metal interconnect layers (e.g., Metal 1, Metal 2, or, if utilized, Metal 3 layers) of the photosensor's integrated circuitry. Light shields formed in such upper fabrication layers, however, have inherent drawbacks. For example, metallization layers dedicated to light shielding are limited in their normal use as conductive connections for the imager array. Additionally, light shields formed in upper device layers are separated from the light-admitting surface of the photosensor by several light transmitting layers. Moreover, the light shields are imperfect, and allow some light to pass into the light transmitting layers. Consequently, optical crosstalk still occurs through the light transmitting layers between the photosensor and the light shields. Having the light shields spaced apart from the surface of the photosensor can also increase light piping and light shadowing in the photosensors, leading to further errors in imager function.
Solid state imagers would benefit from a more efficient method of trapping incident light to reduce optical crosstalk.